Axillary infrared thermometer and method of use

ABSTRACT

A radiation detector for axillary temperature measurement comprises a wand having an axially directed radiation sensor at one end and an offset handle at the opposite end. The radiation sensor is mounted within a heat sink and retained by an elastomer in compression. The radiation sensor views a target surface through an emissivity compensating cup and a plastic film. A variable reference is applied to a radiation sensor and amplifier circuit in order to maintain full analog-to-digital converter resolution over design ranges of target and sensor temperature with the sensor temperature either above or below target temperature.

RELATED APPLICATIONS

[0001] This application is a Continuation of 09/498,279 filed Feb. 4,2000, which is a Continuation of Ser. No. 09/253,970 filed Feb. 22,1999, now U.S. Pat. No. 6,045,257, which is a Divisional of Ser. No.08/738,300 filed Oct. 25, 1996, the entire teachings of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Neonates are incapable of maintaining their own body temperatureduring the first few weeks of life. Skin perfusion rates are very highand the infant loses heat rapidly. Thermal management is critical,requiring an accurate, fast, noninvasive method of core temperaturemeasurement.

[0003] Rectal temperature has long been considered to be the standardindicator of neonate core temperature. However, since temperaturemeasurements from different locations on a neonate's skin aresufficiently uniform as to be relatively interchangeable with oneanother, the clinician may select the most noninvasive and convenientsite at which to measure temperature. Due to its inherent safety andlong established efficacy, axilla (underarm) is the most recommendedsite for neonates. Unfortunately, conventional thermometers such asglass/mercury, electronic and paper strip thermometers require up toseveral minutes to obtain an accurate axillary reading.

[0004] In recent years, infrared thermometers have come into wide usefor detection of temperature of adults. For core temperature readings,infrared thermometers which are adapted to be inserted into thepatient's ear have been extremely successful.

[0005] Infrared ear thermometry has not found such high acceptance foruse with neonates. Neonates have a very high moisture level in their earcanals, due to the presence of vemix and residual amniotic fluid,resulting in low tympanic temperatures because of the associativeevaporative cooling. In addition, environmental uncertainties, such asradiant heaters and warming pads can significantly influence the airtemperature. Further, clinicians are less inclined to position the tipof an infrared thermometer in the ear of a small neonate.

[0006] An infrared thermometer designed for axillary temperaturemeasurements is presented in U.S. patent application Ser. No.08/469,484. In that device, an infrared detector probe extends from atemperature display housing and may easily slide into the axilla tolightly touch the apex of the axilla and provide an accurate infraredtemperature reading in as little as one-half second.

[0007] The axillary infrared thermometer has found great utility notonly with neonates but as a screening tool in general and especially forsmall children where conventional temperature measurements such as athermometer under the tongue or a rectal thermometer are difficult. Theaxillary measurement is particularly accurate with children where thereis a lack of axillary hair and perspiration.

SUMMARY OF THE INVENTION

[0008] The present invention has particular applicability to an axillaryinfrared thermometer which is suitable for nonclinical use but whichretains the accuracy required for clinical use.

[0009] In accordance with one aspect of the invention, the thermometercomprises an extended housing forming a wand which has a handle portionat one end and a radiation detection portion at the other end. The wandconfiguration allows the device to be passed through the sleeve of thepatient to the axilla to minimize exposure of the patient to thesurrounding environment. Further, the housing surface is formed of a lowthermal-conductivity material to minimize cooling of the patient withcontact against the skin. The radiation detection portion of the housingcomprises an axially directed window positioned within a cup. The windowpasses radiation to an infrared radiation sensor within the housing froma field of view substantially less than the area of the cup opening.

[0010] In accordance with another aspect of the invention, the handleportion of a radiation detector housing, preferably configured as awand, extends along an axis generally parallel to the viewing axis butoffset from the viewing axis.

[0011] In accordance with another aspect of the invention, the cup isdesigned to conduct heat from the target area in order to match the heatloss from the target area when the radiation detector is not in place,thereby minimizing changes in target temperature.

[0012] In accordance with another aspect of the invention, the radiationsensor is mounted within a large thermal mass within the lowconductivity housing to provide an RC time constant for change intemperature of the radiation sensor, with change in temperature to whichthe housing is exposed, of at least 5 minutes and preferably 25 minutes.

[0013] In accordance with another aspect of the invention, a transparentplastic film is positioned over the cup to minimize the effects ofevaporation from the target surface. The thickness of the film is atleast 1.1 mils. and is preferably 1.5 mils.

[0014] In a preferred configuration, the radiation sensor is mountedwithin a can for viewing a target through an infrared transparent windowon the can and through an aperture at the base of an emissivitycompensating cup. The can is mounted within a bore in a heat sink withthermal coupling to the heat sink through a rear flange. The flange maybe pressed against a shoulder by an elastomer such as an o-ring. A rearcap fit to the heat sink presses against the elastomer to press the canagainst the shoulder. Preferably, the cap is an externally threaded plugwhich fits within the bore. The cap has an opening therein through whichelectrical leads to the radiation sensor pass.

[0015] In accordance with another aspect of the invention, a noveldetection circuit is provided. A radiation sensor provides an output asa function of difference between target temperature and sensortemperature over a design range of target temperatures and a designrange of sensor temperatures. An amplifier amplifies the sensor outputand an analog-to-digital converter generates a multibit digital outputfrom the amplified output over a voltage range of the amplified sensoroutput. A reference to the radiation sensor and amplifier circuit isvariable to provide high analog-to-digital converter resolution, overthe design ranges of target and sensor temperatures, with sensortemperatures either above or below target temperature. The resolution isgreater than would be obtained with a fixed reference over fall designranges of target and sensor temperatures. The variable referenceprovides a variable offset which may be subtracted out in digitalprocessing circuitry prior to digital computation of target temperature.

[0016] In the preferred detection circuit, the reference is variable tooffset the amplified output by an offset level approximating sensortemperature. The amplified sensor output then approximates targettemperature. In an alternative embodiment, the reference is set at oneof two levels depending on whether target temperature is above or belowsensor temperature. In one implementation of that embodiment, thereference is set to one of those two levels only after the amplifiedoutput has exceeded the analog-to-digital converter range using anintermediate reference.

[0017] In each embodiment of the detector circuit, the reference levelis preferably set while also compensating for amplifier offset. Theradiation sensor is isolated from the amplifier and the amplified outputis varied either to approximate the sensor temperature or to set theamplifier output at one of three levels corresponding to the tworeference levels. Preferably, a resistor which balances the resistanceof the radiation sensor is also isolated from the amplifier during theoffset calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0019]FIG. 1 is a cross-sectional view of one embodiment of theinvention.

[0020]FIGS. 2A and 2B are orthogonal side views of another embodiment ofthe invention.

[0021]FIG. 3 is a side view of a preferred embodiment of the invention.

[0022]FIG. 4 illustrates use of the embodiment of FIG. 3 with an infant.

[0023]FIG. 5 is a cross-sectional view of the radiation sensor assemblyin the embodiment of FIG. 3.

[0024]FIG. 6 is an electrical block diagram of one implementation of theinvention.

[0025]FIG. 7 is an electrical block diagram of an improvedimplementation of the invention.

[0026]FIG. 8 is an electrical block diagram of the preferredimplementation of the invention.

[0027]FIGS. 9A, 9B and 9C are detailed electrical schematics of thepower control circuit, ambient temperature detection circuit and sensoramplification circuit, respectively, of FIG. 8.

[0028]FIG. 10A is a flow chart of the measurement process includingoffset definition and FIG. 10B is a flow chart of digital processing toobtain target temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0029] A description of preferred embodiments of the invention follows.

[0030]FIG. 1 illustrates a compact form of a radiation detector designedfor axillary temperature measurement. It includes a conventionalthermopile radiation sensor in which the thermopile is mounted in a can20 having a window 22 through which the sensor views a target. Asillustrated, the field of view of the sensor is about 60° and is througha reflective cup 23 having an angle of about 90°. The housing 24 inwhich the sensor is mounted has a handle portion 26 which extends in adirection generally parallel to the line of sight of the radiationsensor but which is offset therefrom. Accordingly, the sensor end 28 ofthe housing may be readily slid into the axilla with the housing held tothe front of the patient. A liquid crystal display 30 provides atemperature reading after a button 32 is pressed. The radiation detectoris powered by a single battery 34.

[0031]FIGS. 2A and 2B illustrate another embodiment in which thedetector housing takes the form of an extended wand. The same radiationsensor views the target through a cup 36 at a distal end of the housing38. The handle portion 40 is again offset from the viewing axis of thecenter 20 and extends along an axis which is generally parallel (i.e.,within about 20°) to the line of sight of the radiation sensor, but inthis case the handle portion is at a proximal end of the extended probe.This design facilitates insertion of the radiation sensor end of thehousing through the sleeve of a patient to minimize heat loss from thepatient during the measurement. To that end, the wand is about 10-20centimeters long. As before, a liquid crystal display 30 provides atemperature reading after the button 32 is pressed. Again, the radiationdetector is powered by a single battery 34.

[0032]FIG. 3 illustrates a preferred embodiment of the radiationdetector. Again, the housing takes the form of an extended wand having aradiation sensor at a distal end thereof which views the target along aviewing axis 44. A handle portion 46 is provided at the proximal end ofthe wand and is again offset from the viewing axis 44. Again, the handleportion extends along an axis which is generally parallel to the viewingaxis 44, though angled about 10° from true parallel. The handle portion46 is joined to the viewing portion 48 along a curved section 50 whichincludes a switch button 52. A reading is provided on a display 54.

[0033] Use of the embodiment of FIG. 3 is illustrated in FIG. 4. As canbe seen, the extended probe can readily be slid into the axilla throughthe child's sleeve. The offset of the handle portion enables it to beeasily held in front of the patient.

[0034] Not only is the use of the wand through the sleeve better for thepatient in minimizing heat loss, it improves the temperature measurementby avoiding cooling of the target area. As disclosed in U.S. Pat. No.5,445,158, the actual temperature of the target surface is offset fromthe desired core temperature by an amount determined by the surroundingambient temperature. Ambient temperature controls the amount of heatloss from the target area and the thermal difference between coretemperature and surface temperature. That temperature difference can becompensated electronically by computing a core temperature T_(C) fromthe sensed target temperature T_(T) and a sensed ambient temperatureT_(A) with a known coefficient K as follows:

T _(C) =T _(A) +K(T _(T) −T _(A))

[0035] However, the degree of compensation can be minimized or eveneliminated if exposure of the target area to ambient temperature isminimized.

[0036] With the wand positioned through the sleeve, the contact of thewand to the axilla is not visible to the clinician. Accordingly, toassure that the actual axilla temperature is obtained, the wand may bepivoted to scan the region, with the electronics providing a peaktemperature detection as in U.S. Pat. No. 5,445,158. In that priorsystem, the peak detection algorithm was processed as long as the buttonon the radiation detector was pressed. In the present system, the buttonis pressed once and the peak detection continues until the temperaturestabilizes. The probe can be scanned with the clinician viewing thetemperature display to locate the location of highest temperature.However, since the detector is surrounded by the target area, scanningbecomes less critical.

[0037]FIG. 5 illustrates the sensor assembly of the radiation detectorof FIG. 3. A thermopile 60 is mounted within a can 62 in conventionalfashion. For high stability the thermopile may be a vapor depositedthermopile surrounded with xenon gas, but for reduced cost it ispreferably a semiconductor thermopile surrounded with air. An infraredradiation transparent window 63 is provided over a viewing opening inthe can. The can 62 is set within a bore within a heat sink 64. Ashoulder defines an aperture 66 at the base of a conical cup 68 throughwhich the thermopile views the target. The cup is preferably of lowemissivity in order to provide emissivity compensation as disclosed inU.S. Pat. No. 4,636,091. Preferably, the heat sink 64 in which the cupis formed is of aluminum. Alternatively, the heat sink may be of brass,nickel plated in the cup region. With the rim pressed against theshoulder and having a close tolerance with the larger diameter of thebore an air gap 73 is maintained about the side and front of the can.

[0038] An elastomeric o-ring 70 is positioned behind the can 62. A plug72 is threaded into the bore in the heat sink 64 to press the ring 70against the rear flange 71 of the can and thus press the flange againsta shoulder in the heat sink bore. This arrangement provides for goodthermal contact between the can and the heat sink 64 at the rear andalso makes the thermopile highly resistant to mechanical shock since theshock is only transferred through the thin rim past the shock absorbingelastomer. If the flange were rigidly clamped between metal parts, therewould be a danger of shock breaking the gas seal of the can. An opening74 is provided through the center of the plug 72 for access ofelectrical leads 75 to the thermopile can.

[0039] The heat sink assembly is mounted to the housing by sliding itinto an opening at the distal end of a front housing section andretaining it with a snap ring 79. Then the rear housing section isconnected to the front section.

[0040] The plastic housing 48 in which the sensor assembly is mounted isof low thermal conductivity, preferably less than one hundredth that ofaluminum. The housing thermally isolates the heat sink 64 from thesurrounding environment to minimize heat flow to the heat sink. Further,the heat sink 64 is of significant thermal mass. Accordingly, the RCtime constant for change in temperature of the radiation sensor, withchange in temperature to which the housing is exposed, can be made largefor a more stable temperature reading. The thermal resistance is madehigh by the low conductivity housing, and the thermal capacitance ismade high by the large mass of the heat sink 64. That RC time constantshould be at least 5 minutes and is preferably about 25 minutes.

[0041] Past designs of infrared thermometers, such as presented in U.S.Pat. No. 4,993,419, have relied on a massive thermopile can which alsoserved as the heat sink. That design assured a high RC time constant forthermal conduction through the external thermal barrier to the heat sinkrelative to a thermal RC time constant for temperature response of thecold junction to heat transferred to the heat sink. The latter low RCtime constant was obtained by assuring a low thermal resistance to thecold junction using expensive high conductivity material in a speciallydesigned can/heat sink. In the present device, a design goal is to use aconventional low cost thermopile mounted in a light weight can whichdoes not provide the low thermal resistance of the prior design.Accordingly, it is important that the can be mounted to assure that allheat conduction to the thermopile be through the rear of the can whichserves as the thermal ground to the thermopile. That objective isobtained by making thermal contact to the can through the rear flangeand assuring an air space about the sides and front of the can.

[0042] Forming the emissivity compensating cup 68 in the heat sinkreduces the cost of the assembly and also improves the thermalcharacteristics. Although the emissivity of the cup is ideally zero, itis in fact about 0.1. With the cup formed as part of the heat sink, itis at the temperature to which the can is grounded. Accordingly, anythermal emissions from the surface 68 will be at substantially the sametemperature as the cold junction and thus not be seen. The electronicscan also be calibrated to compensate for the loss of reflectance due tonon-ideal emissivity, but that calibration is affected by thetemperature of the reflective surface. By assuring that the surface 68is at the temperature to which the thermopile can is grounded, thetemperature of the surface is generally known and compensation can bemade temperature dependent.

[0043] When adapted to household use, concerns for patientcross-contamination associated with clinical temperature detectors isnot so significant. Accordingly, the disposable radiation transparentcovers used in prior infrared thermometers, such as in Ser. No.08/469,484, are not required. However, for purposes of accuracy ofmeasurement, it is still important that a thin transparent film beprovided over the viewing area. Without the film 76, any evaporationfrom the moist axillary region would result in a temperature reductionat the target surface and reduced accuracy in the temperature reading.However, the film can be pressed against the target surface, thustrapping the moisture and preventing evaporation. The thin film quicklyequilibrates to the temperature of the target surface for an accuratereading.

[0044] In prior infrared thermometers in which the film was notcontacted to the target, it was important that the thickness of the filmbe minimized. Such films have generally been described as being of lessthan 0.001 inch thickness, but thicknesses of less than 0.0005 inch havebeen preferred to minimize absorption of the radiation in the film andthe resultant heating of the film which would lead to inaccuratereadings. With the present application, where the film contacts thetarget surface, the film not only allows a change in temperature, butthe temperature is forced to change to the target temperature.Accordingly, absorption of radiation is no longer a concern. Now thelimitation in thickness is the need to obtain a quick equilibration withthe target surface. Further, since the film must be more durable inorder to obtain repeated measurements while contacting the targetsurface, a thickness of greater than 0.0011 inch is important. In thepreferred embodiment, a proper balance between durability and quickthermal response is obtained with a film thickness of about 0.0015 inch.As before, polyethylene is the preferred film material.

[0045] Accordingly, a thin plastic film 76, for example of 0.0015 inchthickness, is stretched across the cup of the heat sink 64. The filmneed only be replaced periodically with wear and may be attached to theend of the housing with a formed sleeve 78 or the like. For example, thesleeve may be injection molded with the film ultrasonically welded toit. The sleeve is sufficiently thick to serve as a thermal insulatorbetween the patient and the heat sink 64.

[0046] Electrical block diagrams for three implementations of theradiation detector are presented in FIGS. 6-8. In each implementation, amicroprocessor 80 is at the heart of the circuit. A power controlcircuit 82 responds to activation of the button switch 84 by the user toapply power to the microprocessor and other elements of the circuit.That power is maintained until the microprocessor completes themeasurement cycle and signals the power control 82 to power down. Themicroprocessor is clocked by an oscillator circuit 86 and maycommunicate with an external source for programming and calibrationthrough communication conductors 88. The temperature determined by themicroprocessor is displayed on the liquid crystal display 100, andcompletion of the temperature processing is indicated by a beeper 102.During the measurement process, the microprocessor takes readingsthrough an internal multiplexer/analog-to-digital converter 90. Thepreferred microprocessor 80 is a PIC16C74 which includes an internal8-bit A-D converter. To minimize expense, the circuit is designed torely solely on that A-D converter.

[0047] With reference to FIG. 6, thermopile 92 provides a voltage outputsignal equal to the fourth power difference between target temperatureand the temperature of the thermopile cold junction, offset by voltagereference 94 to the thermopile. The voltage output from the thermopileis amplified by an amplifier 96, having a gain in the order of 1000,which also provides an offset. Through operation of the multiplexer, themicroprocessor provides an analog-to-digital conversion of the amplifiedsensor output, of the thermopile and amplifier reference voltage 94 andof an ambient temperature provided by temperature sensor 98.

[0048] The microprocessor utilizes the measured voltage reference signal94 to determine and allow for electrical offsets in the circuit. Thetemperature sensor 98 is positioned to sense the substantially uniformtemperature of the thermopile cold junction, can and heat sink. Withlong thermal time constants, that temperature is also considered to bethe same as ambient temperature during the short temperature reading.The microprocessor relies on the sensed temperature T_(A) to computetarget temperature T_(T) from the relationship (T_(T) ⁴-T_(A) ⁴) towhich the thermopile output is proportional. The microprocessor may alsocompute core temperature from the core temperature relationship notedabove.

[0049] ASTM standards dictate that the infrared thermometer operatecorrectly over a range of ambient temperature of 16°-40° C. and provideaccurate temperature readings of target temperatures ranging from22°-40° C. A design goal of the present radiation detector was toprovide accuracy of 0.1° C. over the full 24 degree range of 16°-40° C.Thus, the 8-bit analog-to-digital converter must be able to convertreadings within a 24° range without saturation at either end of thatrange. With a full range of 24° and 0.1° accuracy, 240 analog levels arerequired. The 256 levels processed in an 8-bit conversion would appearsufficient. However, the input to this A-D converter actuallyapproximates the difference between target temperature and sensor(ambient) temperature, and that difference may be up to 24° positive ornegative. Allowing for the target temperature to be either above orbelow the ambient temperature by 24° necessitates a reduction in A-Dconverter resolution by one half in order to convert the full ±24° C.range without saturation of the analog-to-digital converter.

[0050] One approach to obtain the 0.1° C. accuracy over the full 24°temperature range with an 8-bit converter, regardless of whether thesensor temperature is above or below target temperature, is to controlthe value of the voltage reference 94 according to the particularenvironmental conditions. For example, if the readings indicate that thetarget temperature is greater than the sensor temperature, the referenceto the thermopile can be set at zero and the converter can convert thefull 24° range of target temperature above sensor temperature withoutsaturation. If, however, the target temperature were to drop belowsensor temperature with that zero offset, the resultant negative valuewould saturate the analog-to-digital converter at the lower end. In thatcase, it is necessary to provide a sufficient voltage reference 94 tothe thermopile to assure that the decrease in thermopile output withmore negative target temperature does not bring the input to the A-Dconverter to zero. The maximum input to the A-D converter then resultsfrom a zero temperature difference and the zero input to the A-Dconverter results from the maximum temperature difference. The voltagereference is controlled by the microprocessor 80, and the microprocessoris programmed to process the different algorithms depending on therelative temperatures which dictate the voltage reference.

[0051] An alternative approach is to initially set the voltage reference94 midway between the two extreme offset references discussed above. Inthat case, so long as the target and ambient temperatures are within 12°C. of each other, the output from the amplifier 96 is within the fullrange of the analog-to-digital converter. If the microprocessor sensessaturation of the A-D converter, however, it changes the voltagereference to the high or low value accordingly. In each case, themicroprocessor processes the received data according to the referencevoltage that it applies.

[0052]FIG. 7 illustrates an alternative approach by which the output ofthe amplifier 96 can be maintained within the acceptable range of theanalog-to-digital converter for full 0.1° C. resolution regardless ofwhere the target and sensor temperatures fall within the designtemperature ranges and without the need for determining whether thetarget temperature is above or below the sensor temperature. In thisapproach, the sensor ambient temperature T_(A) provides the voltagereference to the thermopile. Thus, if the ambient temperature were at16° C., the minimal offset would be provided to the thermopile 92 andthe output of the thermopile would be proportional to (T_(T) ⁴-T_(A) ⁴)through the full range of possible target temperatures. On the otherhand, if the ambient temperature were at 40° C., the maximum offsetwould be provided to the thermopile and the output of the thermopilewould be proportional to that maximum offset minus a voltageproportional to (T_(T) ⁴-T_(A) ⁴). Accordingly, the microprocessorsubtracts that offset before computing target temperature. If theambient temperature is at any level between the minimum and maximumdesign temperatures, the offset applied to the thermopile is set at acorresponding value between the minimum and maximum offsets. By slidingthe offset with sensor temperature, it is assured that the output ofamplifier 96 is within the 8-bit range of the analog-to-digitalconverter, regardless of the level of the target temperature, so long asthe target temperature is within the design range. The microprocessoronly has to subtract that offset from the sensed voltage prior tocomputing target temperature from the value (T_(T) ⁴-T_(A) ⁴).

[0053] The above discussion presents a design technique which allows forthe highest resolution using the 8-bit analog-to-digital converterregardless of target and ambient temperatures within the 16°-40° C.design temperature ranges. However, the discussion has not taken intoconsideration electrical offsets in the circuitry. It is desirable tominimize the costs of the analog circuitry, but less expensive circuitelements result in substantially higher amplifier offsets. Further, theoffset itself is temperature dependent, and the high gain is applied toany changes in offset. Accordingly, with an inexpensive amplifier,changes in electrical offsets with temperature can alone exceed theavailable input range of the analog-to-digital converter.

[0054] In accordance with the approach of FIG. 8, the voltage offset tothe amplifier is controlled to account for whether the targettemperature is above or below ambient temperature, as discussed above,and also to compensate for wide variations in electrical offset topermit use of relatively inexpensive analog circuitry. The thermopilevoltage reference is fixed at a level that assures that the thermopileoutput is always an acceptable positive input to the amplifier. An autozero switch 104 is included to allow for isolation of the amplifier 96from the thermopile 92 during a calibration sequence. During thatsequence, the microprocessor provides a pulse width modulated output online 106 which is filtered by a filter 108 to produce an offset levelapplied to the amplifier circuit. With the thermopile isolated from theamplifier, the amplifier output can be adjusted using the electricaloffset to set the output of the amplifier 96 at the level dictated bythe considerations discussed above. Specifically, in the first approach,the output of amplifier 96 is adjusted to one extreme of theanalog-to-digital converter range depending on whether the targettemperature is higher or lower than the sensor temperature. In themodification to that approach, the output of the amplifier 96 isinitially set at an intermediate level. In the final embodiment, theoffset applied to the amplifier 96 is selected to set the amplifieroutput at a level which approximates the ambient temperature sensed bysensor 98. In each case, the electrical offset of the amplifier isbalanced out, leaving only the offset required to keep the A-D converterwithin range.

[0055] After calibration, the thermopile is again connected into thecircuit using switch 104, and the output from amplifier 96 becomes thevoltage determined by (T_(T) ⁴-T_(A) ⁴) plus the voltage offset whichhad been set by the microprocessor to maintain full resolution withinthe design temperature range. The target temperature is then computed byfirst subtracting the applied offset and then determining targettemperature with knowledge of the sensor temperature.

[0056] FIGS. 9A-9C present a specific circuit implementation of the FIG.8 approach. The power control circuit is illustrated in FIG. 9A. Whenthe switch S1 is pressed, power from the battery junction J2 is appliedacross diode D1, resistor R10 and zener diode D2 to apply power toterminal VSW. Terminal VSW applies power to the remainder of the system.The battery voltage is also seen by the microprocessor through resistorR1 at control line VSWITCH. The microprocessor then applies a controlsignal to line VON which turns transistors Q2 and Q1 on to maintain VSWhigh even after switch S1 is released. The voltage across zener diode D2is applied to a voltage regulation circuit including amplifier U1A togenerate a 3.5 volt reference required by the microprocessor. A 1.75volt reference used in the amplifier circuit is also generated through atransistor Q5.

[0057]FIG. 9B illustrates the ambient temperature sensor 98 whichactually measures the thermopile cold junction temperature. The 3.5 voltreference from the power control circuit is applied across a thermistor110 in series with resistor R4. The temperature dependent voltage fromthe voltage divider of thermistor 110 and resistor R4 is applied to thenon-inverting input of amplifier U1C to provide a temperature indicationVTA. VTA is converted to a digital signal through the multiplexer/ADconverter 90 of the microprocessor 80.

[0058]FIG. 9C illustrates the thermopile and amplifier circuit includingthe offset control. The output of thermopile 92 is applied to thenon-inverting input of amplifier 96 to produce the signal V_(SIG)applied to the multiplexer/AD converter 90. During calibration, thesignal AZ from the microprocessor through transistors Q7 and Q6 closesthe switch Q3, thus short circuiting the thermopile and isolating itfrom the amplifier 96. Included within the feedback circuit of theamplifier 96 are resistors R17 and R18 which balance the internalresistance of the thermopile and resistor R15, respectively, when thethermopile output is being amplified. During the calibration cycle thoseresistors are also shorted by switch Q4 to maintain balance.

[0059] The pulse wave modulated signal on line 106 of FIG. 8 is appliedas control signal PWM in FIG. 9C. That signal is low pass filtered in anRC circuit (R27, R28, C5, C6) and applied through amplifier U1D tocreate an offset voltage at the output of amplifier U1D. With thethermopile isolated, the microprocessor controls that offset voltage toadjust the output of amplifier 96 to a desired level. Thereafter, themicroprocessor maintains that offset voltage level. That offsetcompensates for the undesirable electrical offset of the amplifier andalso creates the electrical offset needed to maintain the signal V_(SIG)within the range of the A-D converter. Thereafter, the microprocessorchanges signal AZ to open the switches Q3 and Q4 and allow detection ofthe voltage signal generated by the thermopile 92.

[0060] Operation of the microprocessor in controlling the calibrationprocess is illustrated by the flow chart of FIG. 10A. The processing ofthe detected signal on line V_(SIG) is as illustrated by the flow chartof FIG. 10B.

[0061] As illustrated in FIG. 10A, the ambient temperature is measuredat 100 by converting the output V_(TA) from the sensor circuit 98 ofFIG. 9B. The switches Q3 and Q4 are closed at 102. With no offset signalapplied through PWM, the output of the amplifier 96 is the unwantedelectrical offset which must be balanced out. That signal is convertedas V_(SIG) Offset at 104. If V_(SIG) Offset is within the range of theA-D converter, it can be measured, and the process continues from 106.On the other hand, if the analog-to-digital converter saturates, it isnecessary to apply a signal PWM to add an offset to the amplifier whichbrings the V_(SIG) Offset signal to within range. Accordingly, theoffset required to change the V_(SIG) output by one-half the A-Dconverter range is applied at 108. The system loops through steps 106and 108 as required to bring the V_(SIG) Offset into a measurable range.

[0062] At 110, the PWM signal required to make V_(SIG) Offset equal toV_(TA) is calculated and applied to the circuit of FIG. 9C. At thispoint, V_(SIG) approximates V_(TA) so the signal should remain withinrange even when the thermopile is switched back into the circuit.

[0063] V_(SIG) Offset is measured again at 112. The calculated V_(SIG)Offset was only an approximation to assure that the A-D converterremains within range. The critical value for later computation of targettemperature is the actual offset measured at 112.

[0064] The FET switches Q3 and Q4 are then opened at 114 and V_(SIG) ismeasured at 116. Temperature is calculated and displayed at 118 as thedetector stabilizes. Until the V_(SIG) signal stabilizes to a constantlevel, the system loops from 120 to remeasure V_(SIG).

[0065] Once the temperature reading has stabilized, the FET switches Q3and Q4 are again closed at 122 and V_(SIG) Offset is measured at 124.This assures an accurate target temperature calculation in the event ofany drift in V_(SIG) Offset during the measurement process. The finaltarget temperature is calculated and displayed at 126.

[0066] The target temperature calculations performed at 118 and 126 arepresented in FIG. 10B. The voltages V_(TA) and V_(SIG) are measured at128. A thermistor offset value stored during manufacturing calibrationis subtracted from V_(TA) at 130 and the resultant value is multipliedby 3.5 volts per 256 bits at 132 since the ambient temperature look-uptable is based on a 3.5 volt range. The look-up table is addressed at134 to obtain the ambient temperature value T_(A).

[0067] A temperature coefficient Tempco which allows for the temperaturedependence of the thermopile output is calculated at 136:

Tempco=(1+K _(T)(T _(A) −T _(Z))

[0068] where K_(T) is a stored sensor specific gain constant and T_(Z)was the temperature of the device stored during manufacturingcalibration.

[0069] At 138, the measured V_(SIG) Offset is subtracted from V_(SIG) toobtain a value which indicates the value (T_(T) ⁴-T_(A) ⁴). Morespecifically, Hc/s is computed at 140:

Hc/s= (V _(SIG) −V _(SIG) Offset)×Kh×(Tempco)

[0070] where

[0071] Kh=Sensor Gain Constant (Determined at calibration)

T _(T)=(Hc/s+T _(A) ⁴)^(¼)

[0072] To obtain target temperature, T_(A) ⁴ is added to Hc/s at 142 andthe fourth root is obtained by table look-up at 144.

[0073] Each of the look-up tables may be modified to account forcompensations such as core temperature compensation and temperaturedependant emissivity calibration as discussed above.

[0074] While this invention has been particularly shown and describedwith references to preferred embodiments thereof it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A radiation detector comprising: a radiationsensor which provides an output as a function of difference betweentarget temperature and sensor temperature over a design range of targettemperatures and a design range of sensor temperatures; an amplifier incircuit with the radiation sensor which amplifies the sensor output; ananalog-to-digital converter which generates a multibit digital outputfrom the amplified output over a voltage range of the amplified sensoroutput; and a variable reference to maintain analog-to-digital converterresolution over the design ranges of target and sensor temperatures, theresolution being greater than would be obtained with a fixed referenceover a full design range of target temperature and a design range ofsensor temperatures.
 2. A radiation detector as claimed in claim 1wherein the reference is variable to offset the amplified output by anoffset level approximating sensor temperature.
 3. A radiation detectoras claimed in claim 2 wherein the amplified output approximates targettemperature.
 4. A radiation detector as claimed in claim 2 furthercomprising a switch to isolate the radiation sensor from the amplifier,the reference being varied to provide an amplified output whichapproximates sensor temperature while the radiation sensor is isolatedfrom the amplifier.
 5. A radiation detector as claimed in claim 4further comprising a switch to isolate a resistor which balances theresistance of the radiation sensor.
 6. A radiation detector as claimedin claim 1 wherein the reference is set at one of two levels dependingon whether target temperature is above or below sensor temperature.
 7. Aradiation detector as claimed in claim 6 further comprising a switch toisolate the radiation sensor from the amplifier, the reference beingvaried to provide an amplified output at one of the two levels while theradiation sensor is isolated from the amplifier.
 8. A radiation detectoras claimed in claim 7 further comprising a switch to isolate a resistorwhich balances the resistance of the radiation sensor.
 9. A radiationdetector as claimed in claim 1 wherein a measurement offset resultingfrom the reference is subtracted from the multibit digital output priorto digital computation of target temperature therefrom.
 10. A radiationdetector as claimed in claim 1 , wherein the variable reference isapplied to the amplifier.
 11. A method of detecting temperaturecomprising: amplifying the output of a radiation sensor with anamplifier and applying the amplified output to an analog-to-digitalconverter; and varying a reference to maintain analog-to-digitalconverter resolution over design ranges of target and sensortemperatures, the resolution being greater than would be obtained with afixed reference over a fall design range of target temperatures anddesign range of sensor temperatures.
 12. A method as claimed in claim 11wherein the reference is varied to offset the amplified output by anoffset level approximating sensor temperature.
 13. A method as claimedin claim 12 wherein the amplified output approximates targettemperature.
 14. A method as claimed in claim 12 further comprisingvarying the reference by isolating the radiation sensor from theamplifier and, while the radiation sensor is isolated from the amplifierbearing the reference to provide an amplified output which approximatessensor temperature.
 15. A method as claimed in claim 14 furthercomprising isolating a resistor which balances the resistance of theisolated radiation sensor.
 16. A method as claimed in claim 11 whereinthe reference is set at one of two level depending on whether targettemperature is above or below sensor temperature.
 17. A method asclaimed in claim 16 wherein the reference is varied with the radiationsensor isolated from the amplifier to provide an amplified output at oneof the two levels.
 18. A method as claimed in claim 16 wherein thereference is initially set at an intermediate level and, if theamplified output is out of range of the analog-to-digital converter, thereference is then set at one of said two levels.
 19. A method as claimedin claim 11 wherein a measurement offset resulting from the reference issubtracted from the multibit digital output prior to digital computationof target temperature therefrom.
 20. A method as claimed in claim 11further comprising the step of varying the reference to the amplifier.